1. Field of the Invention
The present invention relates to a semiconductor light amplifier used in a light communication system, and a method for light amplification by the light amplifier.
2. Related Background Art
In general, a semiconductor light amplifier refers to one which has a semiconductor laser structure and amplifies an external input light with a biasing current lower than a threshold. In the field of light communication, methods have been developed to compensate for light loss which occurs in the connection in a fiber or between fibers. Further, since the semiconductor light amplifier has a gain over a wide wavelength range and can simultaneously amplify light having different wavelengths, it is expected for use in simultaneous amplification of multi-wavelength signals.
Where the semiconductor light amplifier is used for the simultaneous amplification of the multi-wavelength signals, it is desirable to impart the same gain to various input light. However, it is known in the prior art semiconductor light amplifier that the gains are different from wavelength to wavelength. Such differences in the gains are harmful in constructing a multi-wavelength communication system.
For example, after a signal light has passed through the semiconductor light amplifier, the output light levels of the signals of different wavelengths are different and hence an additional requirement such as broadening the dynamic range of a light receiver is imposed.
On the other hand, based on the restriction imposed to the light receiver, the wavelengths of the signal lights, the wavelength multiplexity and the wavelength range are determined. This is disadvantageous in designing the system.
The wavelength dependency of the gain, that is, the gain spectrum is mainly due to a laser structure. Accordingly, the gain spectrum is designed by designing an activation layer of the semiconductor laser structure.
As a method for overcoming the above problems and improving the gain spectrum, Japanese Laid-Open Patent Application No. 1-179488 discloses a method of imparting a quantum well structure having different quantum wells to a light amplifier. On the other hand, U.S. Pat. No. 4,680,769 discloses a method for constructing a laser structure by connecting two serially arranged activation layers. In those methods, the gains are superimposed on the spectrum to design a gain spectrum in a desired wavelength range, particularly in a wide wavelength range.
However, in those methods, the design is relatively complex because the activation layer itself is a parameter to determine the gain spectrum. For example, in the method disclosed in Japanese Laid-Open Patent Application No. 1-179488, a desired result is not readily attained unless the design is made with the consideration of a current injection condition.
In the method disclosed in U.S. Pat. No. 4,680,769, it is difficult to connect the two activation layers and it is necessary to pay full attention to an optical alignment.
Those methods may be effective when the semiconductor light amplifier is designed for a wide wavelength range of several tens to several hundreds nm, but since only one activation layer may have a broad gain spectrum distribution, it is not always wise to try to attain the equal effect in a relatively narrow wavelength range. In such a case, the superposition of the gain spectra described above may cause the degradation of other properties such as increase of noise.